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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP electron microscopy29,30 enable such comprehensive experiments, capable of mapping the interplay of electronic, structural, electrochemical, mechanical, and thermal kinetics of degradation phenomena. VISUALIZING IN SITU DENDRITE GROWTH WITH MAGNETIC RESONANCE IMAGING Lithium metal anodes have arguably been a holy grail for battery technology, promising ten times higher charge storage capacity than standard graphite anodes and significantly higher energy density for conventional lithium-ion batteries and next generation lithium-sulfur and lithium-air batteries. The quest for viable Li metal anodes has been thwarted by dendrites, multi-branching tree-like structures that nucleate on the anode, grow across the electrolyte to the cathode on repeated charge-discharge cycling, and thereby short circuit the battery—a serious safety risk with flammable organic electrolytes. Enormous effort has been devoted to solving the dendrite challenge since their discovery in lithium batteries in 1976, so far without success.53 Magnetic resonance imaging based on the nuclear magnetic resonance of protons (1H) in the liquid organic electrolyte surrounding the dendrite offers an innovative way to watch dendrites form and grow with higher space and time resolution than previously available.24 Earlier approaches used nuclear magnetic resonance of 6LI and 7Li, which are only marginally sensitive to distinguishing surface and bulk morphology. The nuclear magnetic resonance of protons in the electrolyte, in contrast, is highly sensitive to the neighboring dendrite surface through shifts in the local steady magnetic field and the radio-frequency field. Isotropic resolution of 180 μm in a 16-min 40-s scan allows real-time 3D tomography, producing movies of dendrite nucleation and growth (see movie S1 in Ref. 24). Such high resolution in situ characterization tools offer the promise of observing in 3D and understanding dendrite nucleation on the otherwise flat anode surface and preferential growth of the dendrite tip, the two key phenomena enabling dendrite formation and growth. (Left) Reconstructed variation of the nuclear magnetic resonance signal around a growing dendrite: (B and C) amplitude |SGE| and (E and F) phase φ(SGE). (Right) Reconstructed dendrite morphology, showing the branching and twisted growth path from anode (bottom) to cathode (top). Image from A.J. Ilott et al., Real-time 3D imaging of microstructure growth in battery cells using indirect MRI, PNAS, 2016, 113, 10779-10784. BC x position EF x position y position y position 2 1 0 π/2 0 -π/2 Rare and Localized Events: Often, degradation and failure are associated with rare and localized events that initiate the process. Dendrite nucleation is an example, an event that cannot yet be predicted and whose origin remains unclear.31-33 While dendrite nucleation itself is not immediately harmful to battery operation, the inexorable growth of the dendrite on charging and discharging eventually extends the conducting dendrite across the liquid or through the solid electrolyte, reaching the cathode and short circuiting the battery.34,35 As another example, fracture is a sudden local event releasing accumulated strains that may have built up over 72 PRIORITY RESEARCH DIRECTION – 5 |SGE| φ(SGE) z position z positionPDF Image | Next Generation Electrical Energy Storage
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